As ICs have increased in size to include larger number of circuit elements, the geometry of the circuit elements has decreased in order to maintain the overall size of the IC relatively small. With decreasing geometries of the circuit elements, providing adequate levels of ESD protection has become increasingly more difficult. In MOS circuits the gate oxide thickness has decreased to below 1000 nanometers (nm), and breakdown voltages are often less than 10 volts. Fowler Nordheim (FN) tunneling through the oxide can occur as low as 7 volts. Device junction breakdown voltages, which are often used to protect the sensitive gate oxide directly, or to trigger a protection structure such as a snap-back device, have remained high to minimize hot carrier generation. In many cases, the minimum junction breakdown voltage is above the gate breakdown voltage. Supply voltages have also been reduced. For circuit devices having geometries down to 0.8 microns, supply voltages have been held at 5 volts. However, below that level, either dual supply (5 volts and 3.3 volts) supplies, or a single 3.3 volt supply have been used. The 3.3 volt supply can be as high as 4.5 volts for burn-in.
Lower voltage clamping or triggering structures, such as zener diodes, have been used for ESD protection. Such devices have been made using the lightly doped drain (LDD) diffusions and heavier source/drain diffusions of the MOS transistors of the IC to form zener diodes in the 6-8 volt range. However, since FN tunneling occurs around 7 volts, these zener diodes do not provide adequate I/O and gate protection for large ESD pulses if used to trigger larger energy handling circuits. Lower voltage diodes cannot be easily made without adding additional process steps, and would tend to be leaky due to band to band tunneling.
Attempts have been made to provide ESD protection using a series of stacked diodes. However, these suffer from a basic problem relating to the temperature coefficient of the diodes. The temperature coefficient of a single diode is about -2 mv/.degree. C. Over the normal operating temperature range of -55.degree. C. to 125.degree. C., the change in voltage is about 140 mv. For a ten diode stack, the change would be 1.4 volts. This change is sufficient to make a compromise between low leakage at high temperatures and adequate voltage protection margin at low temperature virtually impossible. Therefore, an alternate approach is necessary.